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Over-mulched trees
Trees
with too much mulch piled on the trunk (right photo) can develop problems at the base of the
trunk. It is best to keep mulch 12" away from the base. If you
MUST mulch this area apply no more than about one inch on the soil.
As a result of too much mulch trunk bark can rot, rodents can burrow resulting
in damage to the bark, roots can grow in the mulch and around the trunk, and
roots are encouraged to cross over each other. These can cause trees to decline and die. Remove mulch from near the trunk so it remains dry to discourage these potential problems (See: removing excess mulch). 
The photo to the right shows a root diving into the soil. This probably resulted from the root growing against the side of the container in the nursery. The mulch becomes a convenient way to hide this potential problem. Recent research shows that diving roots (roots that grow downward after being deflected by the container wall) make the tree unstable if there are no lateral roots nearby to stabilze trees.
Treatment: Cutting the main root just before it dives is a good solution. Bury the cut root segment with at least two inches of soil and mulch to keep it moist to encourage new root formation.
